Radio signaling system



Aug. 13, 1940. J. w. CONKLIN RADIO SIGNALING SYSTEM Filed Jan. 29.. 1938 Patented Aug. 13, 1940 PATENT OFFICE namo SIGNALING SYSTEM James W. Conklin, Audubon, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 29, 1938, Serial No. 187,618

Claims.

My invention relates to modulating systems, and more particularly to a modulating system for a high frequency television transmitter which consists of varying the internal impedance of 5 modulator tubes connected across the load terminals of a tuned quarter wave transmission line which is so connected that the input terminals of said line effectively vary the impedance across an output load connection such as an antenna.

The prior art discloses many ways for modulating high frequency broadcast transmitters. v However, the wide range and high frequency of the modulation which is necessary in a television transmitter makes methods usually applicable to normal broadcast frequencies unsatisfactory for this purpose. For example, the widely separated side band frequencies would be seriously attenuated by the successive resonant circuits usually employed in broadcast transmitters after the point of modulation.

The modulator of the present disclosure broadly relates to the principle concerned in a patent to Crosby, No. 2,085,418, issued June 29, 1937, except that in the instant case a transmission line which is an efiective odd number of quarter wave lengths in length is utilized in place of the artificial line or wave filter of Crosby. A further improvement is here included relating to the variable impedance tubes terminating the .line.

A serious objection to the method taught by Crosby and the prior art is that the line terminating impedance is limited by the capacitive input impedance of the thermionic tubes used to load the line. This becomes increasingly more serious as the frequency is raised, and at'ultra high frequencies prevents the carrier from being fully modulated. While it might at first appear that the tube input capacitance might be efiectively eliminated by creating an anti-resonant circuit 40 with a shunt inductor, it was discovered that the energy stored in such a circuit tends to limit the rate at which the-current-voltage relations can vary. Consequently, since the instantaneous current-voltage relation determines the impedance which is reflected across the load,-

high frequency modulation is seriously impaired. My invention not only eliminates this difliculty but also results in improved operation in a manner to be hereinafter described.

It is an object of my invention, therefore, to provide improved means for modulating a high frequency carrier over a wide range of modulation frequencies without side band attenuation.

It is another object of my invention to provide a high frequency transmitter'which may be modulated by relatively high frequency signals with,- out attenuation of the side bands.

It is another object of my invention to provide means whereby water-cooled thermionic tubes of large dimensions may be satisfactorily employed 5 as modulator tubes at ultra high radio frequencies.

It is another object of my invention to provide means for introducing high voltage regulation between a source of carrier frequency potential 10 and the modulated load circuit.

It is another object of my invention to provide self compensation for the effect of detuning the carrier frequency source under the varying load conditions occurring during the modulation 15 cycle.

A further object is to provide means for modulating a radio frequency carrier which favors the higher modulating frequencies and so compensates for normal high frequency attenuation 20 in the modulator amplifier.

A still further object of my invention is to provide means for utilizing the impedance-inverting characteristic of a quarter wave resonant line for the purpose of modulating a radio fre- 25 money carrier.

Another purpose is to provide means for connecting one or more thermionic tubes across a quarter wave line so that a very high impedance is presented to the line by the tube or tubes. 30

A further purpose is to provide means for varying the impedance of a tube so connected in accordance with a modulating signal.

A further object is to provide a simplified modulating device for a high frequency transmitter. 35

My invention will be better understood from the following description when considered in connection with the accompanying drawing. Its scope is indicated by the appended claims.

Referring to the drawing, I have shown a cir- 40 cult diagram showing one embodiment of my invention suitable for use with a television transmitter.

A source of radio frequency energy is indicated at i, which may' be an oscillator or a power amplifier. The output from this source is coupled by any convenient means to a first transmission line 5|- comprising conductors 3 and 5 which are preferably. hollow copper tubes substantially a quarter wave length long. j It is well 60 known that a quarter wave line may be used to couple unequal impedances which-are connected across its terminals. This characteristic is utilized in my invention to couple the relatively high impedance source of carrier frequency to the relatively low impedance of a load, which may be an antenna, a'transmission line, or any other utilization device, the impedance of which is represented in the diagram by resistor IS. A further well-known characteristic of a quarter wave line is that when a constant voltage is applied to its input, the current through its load is constant regardless of the value of the load impedance. Where the conditions at the load are represented by the formula E=IZ, if I is constant it is evident that E must vary in proportion to Z, which is the loadimpedance. Voltage variations across an antenna cause variations in the radiated en- .ergy, which is the condition obtained in amplitude modulation. It follows therefore that in a circuit of this nature amplitude modulation may be accomplished by varying the load impedance of a quarter wave transmission line. While I prefer to use a quarter wave line, the operation of my device and the theoretical discussion here presented is not limited to such a line, but may also be applied to a line any odd number of quarter wave lengths long, such as A or At ultra high frequencies, the length of a quarter wave line may be such as to make it desirable to use a wave line, or even longer.

A quarter wave line, therefore, in effect provides regulation between a constant voltage input and its load. If this were not so, no modulation could be accomplished, for the constant input would.

appear as a constant voltage across the load, and

the current would'vary with the load. A series resistance between a constant voltage source and a variable load serves a similar purpose, but consumes energy. The quarter wave method here ative values of the terminating impedancs. In

the instant case, a relatively high voltage, is impressed across the high impedance input to the line and consequently a relatively low voltage appears across the low impedance represented by the antenna.

A second quarter wave line 53 consisting of two similar hollow conductors I and 8 is connected at an angle to the first quarter wave line 5| at the junction of 5| and the load impedance IS. The second line has a shorting bar ll connected across its extremities. Consequently there is a voltage node at this point, and a voltage maximum at the point of junction with the first line, which results in its presenting a very high impedance across the load. The mid-point of shorting bar H is at zero radio f requency potential, and is consequently grounded.

A third quarter wave line 55 is now to be considered. Like the first two, it also consists of two hollow copper conductors 45 and 41 which are connected at the junction point of the first and second lines to form an extension of the former. The cathode leads 5'! and 59 may be consideredas an extension of conductors 45 and 41, and thus a part of the resonant circuit 55. This is true because the cathodes are fioatingj that is, they are ungrounded and carrier freq ency potentials appear on them out of phase with respect to each other.

Two modulator tubes 2| and 23 are connected across the extremities of the third quarter wave line 55. These tubes are preferably of the watercooled, external-anode type, such as the RCA 891. Their anodes 25 and 21 are connected together and grounded. This serves two important purposes. The first is that no insulation is necessary for the cooling system. The secondis that, because of the construction which commonly makes the anode the outer shell of the tube, a very short ground connection is obtainable.

The electron-emissive cathodes 33 and 35 are energized by a source I3 which is preferably an A. C. or D. C. generator. One terminal of the generator is grounded. The first terminal of cathode 35 is connected to the remaining generator terminal by a conductor 11 which is within the hollow tubes 9 and 45. The first terminal of cathode 33 is similarly connected to the generator by a conductor [5 within the'hollow members I and 41. The same terminal of each cathode is also bypassed to the extremities of line 55 by two capacitors 31 and 39. The remaining terminal of each cathode is connected to said extremities of line 55, thus completing the cathode heating circuit to generator I3 by means of the parallel members of lines 53 and 55.

The two grids 29 and 3! are connected together by means of an inductor 43, which has such a value that at the carrier frequency a shunt anti-resonant circuit is formed with the grid-anode capacity of tubes Hand 23. The result of this is that at the carrier frequency there is an extremely high impedance between grid and anode, between grid and ground, and betweencathode and ground.

It is this feature of my invention which makes possible the wide variation of terminal impedance which has not heretofore been obtained. The tube impedance, represented by the cathodeground. impedance, approaches infinity in the manner of an anti-resonant circuit, and is no r longer limited by the impedance represented by the interelectrode capacities.

The potential of each cathode, as pointed out above, is oscillating at carrier frequency, therefore a voltage is induced on each grid equal to the instantaneous potential of its associated cathode multiplied by the ratio of the grid-anode impedance to the cathode-anode impedance. This value is slightly less than unity because the antiresonant circuit makes the grid-anode impedance extremely high with respect to the grid-cathode impedance. Thus the grid potential at any instant approximately equals the associated cathode potential.

A mid-tap connection 49 is provided on inductor 43 which is connected to one terminal of a source of modulation frequency 4 I, preferably a video amplifier. The remaining terminal of said source is connected to ground through a bias battery 6| Thus when a positive impulse of a modulating voltage from 4| is superimposed on the radio frequency potential of the grids 29 and 3| inphase, they are thereby made positive with respect to their associated cathodes without interfering with the instantaneous carrier frequency oscillations. The tube impedance therefore becomes very low across the output terminals of quarter wave line 55. Line 55 is thus equivalent to line 53 and its loading effect across 19 is negligible. On the other hand, if a negative impulse of modulating voltage is superimposed on the grids,

they become negative with respect to their cath-' odes and the tube impedance across the terminals of line 55 will become very high. The result of this is that the reflected and inverted load across the load impedance Is now becomes very great,

amounting practically to a short circuit. That this is so may be seen from the discussion on page 272 of Everetts Communication Engineering, second edition. in which the statement is made erably so chosen that, with no modulation, the grids bear such a relation to their associated cathodes that the shunt impedance presented by the line 55 equals the antenna impedance IS. The total power is then divided equally between the antenna and the modulator tubes. When modulation is applied, the eii'ective impedance of i9 varies between zero and twice its no-modulation value. -The voltage across it varies similarly. Therefore, the modulator tubes must be capable of absorbing half the radio frequency carrier output in the absence of modulation. During negative modulation peaks the antenna is short circuited and the load impedance presented to the amplifier tubes is so high that substantially no power is delivered to the load or the modulator tubes. will likewise absorb no power at all from the antenna since they present an extremely high shunt impedance to the antenna.

It has been found that this system favors high modulating frequencies and consequently. compensates for the usual amplifier characteristic which tends to attenuate them. That this is so is evident from a consideration of the effect of the modulating frequencies upon the grid potentials. The energy stored in the grid resonant circuit, comprising inductor 43 and the grid anode capacity of the modulator device, tends to maintain the grid voltage at a constant level. The rate of discharge of energy from this tuned circuit, as is well known, is determined by its decrement. So long as the rate of change of the modulating potential applied to the grids is negligible compared to this rate of discharge, it is not aifected by it. However, at higher modulating frequencies, the effective instantaneous grid potential is changed by the. dissipation of energy into the grid from the grid resonant circuit. The effect of this is that the grid can no a longer follow the radio frequency potential of the cathode; that is, the relative voltage between grid and cathode tends to increase. It may be seen that this is equivalent to an increase in the modulating voltage, thus compensating for the inherent decrease in output from the modulating amplifier at the higher frequencies.

Another important feature of my invention is that high "Q resonant circuits may be utilized in the radio frequency amplifier making possible more eflicieut amplification. Prior methods of modulating a radio frequen'cy carrier were always subject to the objection that sharply resonant circuits subsequent to thepoint of modulation tend to attenuate the higher frequency side bands. This is especially serious in a television transmitter for two reasons, In the first place, high frequency circuits are inherently high Q circuits; and, in the second place, a television system necessarily uses high frequency modulation.

While I have shown the cathode circuits energized by means of quarter wave section 53, it is,

On positive peaks the modulator tubes cathodes, or leads may be introduced within quarter wave section 5| at the mid-point of the coupling inductor.

When a radio frequency oscillator is coupled into quarter wave line 5| without'an intervening amplifier, it is possible to utilize another characteristic of such a line to partially compensate for frequency modulation which results from varying the load impedance across that line. A quarter wave line tuned slightly above or below resonance no longer presents a pure resistive load at its input terminals, but in the manner of all tuned circuits becomes reactive on either side of resonance. Furthermore, the value of the reactive component varies with the load impedance, and cunsequently by a proper choice may be made to detune the oscillator in a manner which will tend to compensate for the undesired frequency modulation.

A further important feature of my invention is that an unusually high impedance from grid to ground has been attained by the use of the resonant grid circuit. Without this resonant circuit, the effective tube impedance across the line could never be any greater than that presented by the inherent grid-cathode capacity. At higher frequencies, this impedance is usully quite low, and therefore the reflected impedance across the antenna due to the modulator tubes would be prevented from approaching zero, and the percent modulation obtainable would be seriously restricted. My invention, however, completely eliminates this difiiculty and 100% modulation has been obtained, substantially fiat over a very wide range of modulation.

I have, therefore, shown a system for modulating an H. F. carrier in which the impedance inversion characteristic of a quarter wave line has been used, and I have further'shown a method for increasing the inherent tube impedance of the modulators so that their shunt effect may be varied over a range greater than before possible.

I claim as my invention: 1. In a high frequency modulated carrier system the combination including a source of carrier currents, a utilization device, a first impedance-inverting means interconnecting said source of carrier currents and said utilization device, a

second impedance-inverting means connected across said utilization device, a pair of thermionic tubes having cathodes, grid and anode electrodes terminating said second impedance-inverting means, means anti-resonant at carrier frequency interconnected with said grid electrodes, and means including a source of modulating potentials for varying the impedance of said thermionic tubes whereby the impedance presented to said utilization device by said second impedance-inverting means is varied by said modulating potentials.

2. A high frequency modulated carrier system comprising a source of carrier currents, a load impedance, 9. first transmission line substantially an odd number of quarter wave lengths long for matching the impedance of said sourceto said load impedance, means for causing amplitude modulation ofisaid carrier frequency at said load impedance which includes a second transmission line substantially an odd number of quarter wave lengths long, a pair of thermionic tubes having anode, grid and cathode'electrodes terminating said second transmission line, said cathode electrodes being a continuation of said second transmission line, an inductor interconnected with said 75' 0 grid electrodes being anti-resonant at carrier frequency, and means intermediate the ends of said inductor for impressing modulating potentials on said grids.

3. A high frequency modulated carrier system comprising a source of carrier currents, a load impedance, a first transmission line substantially an odd number of quarter wave lengths long for matching the impedance of said source to said load impedance, means for causing amplitude modulation of said carrier frequency currents at said load impedance which includes a second transmission line substantially an odd number of quarter wave lengths long connected across said load impedance, a pair of thermionic tubes having cathode, grid, and anode electrodes, said second transmission line terminating at said cathode electrodes, said anode electrodes being grounded, an inductor interconnected with said grid electrodes being anti-resonant at carrier frequency, and means intermediate the ends of said inductor for impressing modulating potentials on said grids.

4. A high frequency modulated carrier system comprising a source of carrier frequency currents, a load impedance, a first transmission line substantially an odd number of quarter wave lengths long transferring energy from said source to said load impedance, means for causing amplitude modulation of said carrier frequency currents at said load impedance which includes a second transmission line substantially an odd number of quarter wave lengths long connected across said load impedance, a pair of thermionic tubes having anode, grid and cathode electrodes terminating said second transmission line, means interconnecting said second transmission line and said cathodes whereby said cathodes assume the instantaneous potential of the adjacent portion of said second transmission line, means grounding said anode electrodes, an inductor interconnected 'with said grid electrodes which is resonated at carrier frequency by the inherent shunt grid-anode capacity, and means intermediate the ends of said inductor for impressing modulating potentials on said grids.

5. In a high frequency modulated carrier system a source of carrier currents, an eifective load impedance, a first transmission line substantially an odd number of quarter wave lengths long for transferring energy from said source to said effective load impedance, means for varying said eifective load impedance in accordance with a modulating signal, said means comprising a second transmission line substantially an odd number of quarter wave lengths long, a pair of thermionic tubes terminating said second transmission line, said tubes having anode, grid and cathode electrodes, means interconnecting said cathodes to said second transmission line, said anode electrodes being grounded, an inductor antiresonant at carrier frequency interconnected with said grid electrodes, and means intermediate the ends of said inductor for impressing in phase modulating potentials on said grids whereby the resultant variation of input impedance of said tubes causes said effective load impedance to vary in inverse proportion.

6. In a device forthe transmission of high frequency modulated signals, a source of carrier currents, an effective load impedance, a first transmission line consisting of hollow tubular conductors substantially an odd number of 'quarter Wave lengths long for transferring energy from said source to said load impedance, means for varying said load impedance in accordance with a signal which includes a second transmission line consisting of a similar pair of hollow tubular conductors substantially an odd number of quarter wave lengths long, a pairof thermionic tubes terminating said second transmission line, said tubes having an electron-emissive cathode, grid and anode electrodes, a source of cathode-energizing potential, a third transmission line consisting of hollow tubular conductors substantially an odd number of quarter wave lengths long connected across said load impedance, means within said second and third transmission lines for interconnecting said cathode electrodes and said source of cathode-energizing potential, means grounding said anode electrodes, an inductor anti-resonant at carrier frequency interconnected with said grid electrodes, and means intermediate the ends of said inductor for impressing modulating potentials on said grids.

'7 In a device for the transmission of high frequency modulated signals a source of carrier currents, an effective load impedance, a first quarter wave transmission line consisting of hollow tubular conductors for transferring carrier current energy from said source to said effective load impedance, means for varying said effective load impedance in accordance with a signal which includes a second similar quarter wave transmission line, a pair of thermionic tubes having electronemissive cathode, grid and anode electrodes, said second transmission line terminating in said cathode electrodes, a source of cathode-energizing potential, a third similar quarter wave transmission line connected across said effective load impedance, means interconnecting the extremities of said third transmission line, means within said second and third transmission lines interconnecting said cathode electrode and said source of cathode energizing potential, means grounding said anode electrodes, an inductor anti-resonant at carrier frequency interconnecting said grid electrodes, and means intermediate the ends of said inductor for impressing modulating and biasing potentials on said grids whereby the effective load impedance is caused to vary in accordance with said modulating potentials.

8. A high frequency modulated carrier system comprising a source of carrier currents, a load impedance, a first resonant transmission line consisting of hollow tubular conductors substantially an odd number of quarter wave lengths long whereby carrier frequency currents are coupled from said source into said load impedance, a variable impedance shunted across said load impedance which includes an impedance-inverting device, and a pair of thermionic tubes having cathode, grid and anode electrodes, anti-resonant means interconnecting said grid electrodes, a source of modulating potential and means including said anti-resonant means and said grid electrodes for varying the tube impedance by said modulating potential, whereby said inverting means impresses said variations across said load impedance.

9. A high frequency modulated carrier system comprising a source of carrier currents, a load impedance, a first transmission line substantially an odd number of quarter wave lengths long for transferring carrier energy from said source to said load impedance, means for causing amplitude modulation of said carrier frequency currents at said load impedance which includes a number of quarter wave lengths long shunted across said load impedance. a pair of thermionic tubes having cathode, grid and anode electrodes, said second transmission line terminating at said cathode electrodes, anti-resonant means interconnecting said grid electrodes, and means including a source of modulating potential for varying the impedance of said thermionic tubes whereby the shunt impedance of said second transmission line across said load impedance varies according to a signal.

10. In a high frequency modulated carrier system, the combination including a source of carrier currents, a utilization device, an impedanceinverting means interconnecting said source oi carrier currents and said utilization device, [a

and anode electrodes,'said tubes being coupled to said utilization device, means anti-resonant at carrierfrequency interconnected with said grid electrodes, and means including a source of modulating potentials for varying the impedance oi said thermionic tubes whereby the impedance presented to said utilization device by said ther- JAMES w; coma-m.

pair of thermionic tubes having cathode; grid 

